1. Field of the Invention
The present invention relates to vacuum engineering, more specifically, to cryogenic sorption pumps, and can be used to produce superclean and oil-free vacuum within a pressure range of 10.sup.2 to 10.sup.-7 Pa while evacuating any gases excepting helium and including corrosive ones from chambers of various designations, measuring from 0.01 to several hundred cubic meters in volume.
2. Description of the Related
There is known a cryogenic sorption pump (SU,A,1333833) comprising a pumping element consisting of a circular vessel containing liquid nitrogen, a porous screen arranged coaxially with the vessel within a space encompassed by its inner side surface, and a sorbent located within the gap between the inner side surface of the vessel and the porous screen.
This pump is disadvantageous in that at the liquid nitrogen temperature the sorbent has a low sorption capacity at low equilibrium pressures (below 10.sup.-3 -10.sup.-4 Pa) of adsorbable gases. As a result, this type of pump fails to provide limiting pressures of below 10.sup.-3 Pa even after a shortterm gas load. To increase the sorption capacity of the pump, the sorbent may be cooled by means of solid nitrogen down to 55.degree.-50.degree. K., but the sorbent cannot be maintained at those temperature for a long time because of high natural heat input to the nitrogen-containing vessel, the nitrogen contents rapidly warming up after evacuation of nitrogen vapors is discontinued. The operation of this pump is hampered by the need for frequently charging the vessel with liquid nitrogen and repeatedly evacuating nitrogen vapors.
Another prior-art cryogenic sorption pump (M. P. Larin, Kondensatsionno-adsorbtsionnaya i sorbtsionnaya otkachka pri temperaturakh tverdogo azota, Zhurnal tekhnicheskoy fiziki, 1988, vol 58, No. 10, October, Nauka Publishers (Leningrad Branch), pp. 2026-2039) comprises a housing complete with a cover fitted with an inlet nozzle for connection of the space to be evacuated and, arranged in the housing, a pumping element and a radiation screen encompassing the pumping element. The pumping element has the form of a circular vessel designed to contain cryogenic agent, with a heat conductor disk welded to its bottom, and heat conductor and porous screen shells arranged coaxially with the vessel and attached to the heat conductor disk. The interspaces between the heat conductor shells and the porous screen shells adjacent thereto are filled with a sorbent material, while the interspaces between the adjacent porous screen shells communicate with the inlet nozzle of the pump.
The radiation screen contains a toroidal vessel adapted to contain cryogenic agent and installed under the pumping element vessel, a shell arranged coaxially with the pumping element, and a chevron screen installed between the pumping element and the inlet nozzle. The lower end of the radiation screen shell is attached in a pressure tight manner to the radiation screen vessel, with the upper end of the shell fitted with a cover connected with the inlet nozzle through a bellows-form heat bridge. The housing is provided with a nozzle for evacuation of the space between the housing and the radiation screen.
The pump also contains a thin-walled pipe installed within the space defined by the inner walls of the radiation screen vessel and having its upper end attached in a pressure tight manner to the cover of this vessel and its lower end attached to the housing bottom. Installed across the pipe in its upper section is a chevron screen having a thermal contact with the radiation screen vessel.
During pump operation the radiation screen vessel is filled with liquid nitrogen while the pumping element vessel is filled with solid nitrogen, i.e. with a cryogenic agent with a lower temperature. The presence of the radiation screen cooled by liquid nitrogen down to 77.4.degree. K. lowers considerably the heat input by radiation from the housing to the pumping element, making it possible for nitrogen to be maintained in a solid state for a long time--scores of times greater than in the case of the pump discussed above, where sorbent cooling is by means of solid nitrogen.
The space between the thin-walled pipe, the inner wall of the radiation screen vessel, the bottom of said vessel, its outer wall, the radiation screen shell, and the pump housing forms a so-called protective vacuum space. Under normal pump running conditions, the pressure in this space is maintained below 10.sup.-4 Pa, providing for greatly reduced heat input by residual gases from the housing to the radiation screen.
In the pump under discussion, however, the annular gap between the radiation screen shell and the pumping element, as well as the space between the heat conductor disc and the cover of the radiation screen vessel, communicate with the inner pump space, thus with the space to be evacuated, so that considerable heat exchange will occur between the radiation screen and the pumping element at working pressures of between 10.sup.2 and 10.sup.-2. This will lead to speeding up the process of solid cryogenic agent and sorbent warming up, hence to shortening the continuous pump operation period prior to nitrogen replenishment in the pumping element vessel and to increasing cryogenic agent consumption rates. All this combines to reduce the pump efficiency and increase labor consumption incident to pump operation.